Fungi Reading Comprehension

MYCORRHIZAS—A TRULY SYMBIOTIC RELATIONSHIP Mycorrhizas is a term used to refer to a symbiotic relationship that forms between a fungus and the roots of a plant (mycos=fungus rhiza=root). In a mycorrhizal relationship, hyphae cover and penetrate the roots and absorb water and minerals which they pass onto the plant. In this way, they become like far-reaching drinking straws for plants. This increases a plant’s ability to withstand drought and ADAPTATION & ENVIRONMENT 48 THEFungus FILES tolerate environmental extremes, which is particularly useful in sandy soils. In addition, the plant receives some protection from soil-borne pathogens including other fungi and nematodes (roundworms). In exchange, the fungi receive a sturdy helping of their host’s food supply—in some cases up to 25% of the total sugars produced! More than 90% of all higher plants have mycorrhizal fungi associated with their roots; in lush lowland forests, the soil is so full of mycorrhizal fungi that they connect the trees together, so trees and their seedlings can exchange food and messages. In the tropics, mycorrhizas are absolutely crucial. This is because there is no true topsoil, rather just rotting leaves and fruits. Trees depend on mycorrhizas to get enough water and nutrients from the soil, so that when forests are cleared, there is no plant debris for fungi to feed on and they die under the hot sun. In recent years there has been a developing interest in using mycorrhizal fungi to establish high-yield planted forests, reclaim stripmined land, and improve growth of important crops. LICHENS AND RADIATION Lichens are sponges of radioactive materials that come to them from the atmosphere, and cause effects all through the food chain. In 1965 a study was launched to examine the effects of radioactive fallout from nuclear bomb tests. The scientists were expecting to use the Sami people as a control group due to their isolated location, but the results showed that the Sami people’s tissues had 55X more radioactivity than the Finns living further south and up to 1/3 of the maximum permissible radioactivity according to the standards of the time. It was found that radioactive cesium and strontium were concentrated by lichens then eaten by reindeer which were then eaten by people. Similarly, after the Chernobyl disaster in 1986, meat from reindeer in Norway and Sweden had 10X the legal limit of measurable radiation and in some areas it exceeded the limit by 20X. The rate of lichen growth is also highly dependent on the condition of the air; where the air is polluted, they will grow very slowly, if at all. Although valuable indicators of ecological health, lichens are also of economic importance to humans. Half of all lichen species tested show some antibiotic activity and some other lichen extracts have been shown to reduce tumours in mice. The lichen Usnea, recognizable as Old-man’s Beard and commonly found hanging from tree branches, has been used medicinally for at least 1000 years. The active compound in this lichen is usnic acid, a potent antiobiotic and antifungal agent that is now marketed under the name Usno. Extracts of Usnea are also widely used in the perfume and fragrance industry. Lichens have also been used through the centuries as a dyeing agent.